This invention relates to a sheet of mica paper composed primarily of synthetic mica platelets impregnated with an organic resin. In a particular embodiment, a plurality of such impregnated sheets are stacked and consolidated to form a laminated board or flexible substrate for a printed circuit. The board may function as a substrate for a printed circuit assembly in which chip carriers and other components are mounted or printed on the substrate. The substrate may also be provided with conductive connections by subtractive or additive circuitry processes.
Sheets of paper may be produced from a variety of different materials. A material is first suspended in liquid as chopped fibers or as a comminuted powder. This forms a slurry or pulp which is then passed through paper making apparatus. Best known are the cellulose papers produced from wood and/or cloth pulps. Where special electrical properties and/or thermal resistance are required, such papers are generally ineffective.
Consequently, considerable effort has been expended in developing papers based on inorganic materials. As reported by the Bureau of Mines in Bulletin 647(1969), entitled "Fluorine Micas", both natural and synthetic micas have been explored at considerable length. See particularly pages 214-242 of bulletin.
Such inorganic papers are produced on a commercial basis, but have never quite fulfilled their promise. Rather, papers and boards presently used for electrical purposes, e.g., component insulation, transformer wrapping and circuit substrates, are largely organics containing fiber fillers and/or fabric reinforcement. Where high temperature substrates are required, sintered ceramics such as alumina are commonly used.
The electrical characteristics or properties required in an electrical material will vary with the application. For example, presently available circuit assembly substrates may provide a low dielectric constant (K) in the range of 4 to 5.5, a dissipation factor less than 0.05, dielectric strength of at least 700 V/mil and volume resistivity of at least 1.0 megohms.times.10.sup.6. In addition, the material should have a flexural strength of at least 10,000 psi, be non-flammable, and be resistant to processing chemicals and to moisture pickup.
Thermal expansion characteristics become important where surface mounting of leadless ceramic chip carriers is required. A particular problem has arisen in many presently available substrates because of high thermal expansion mismatch as well as highly anisotropic expansion coefficients. This means that the actual values of the coefficients along the three axes of a board may vary widely. This is generally believed to result from material orientation and/or tensions introduced by fabrics, such as glass cloth.